ClamXav is a free virus checker for MacOS X. It uses the tried, tested, and very popular ClamAV open source antivirus engine as a back end.
Pros: “Back in the days before OS X, the number of viruses which attacked Macintosh users totaled somewhere between about 60 and 80. Today, the number of viruses actively attacking MacOS X users is … none!” So, unless you’d like to be prepared for any future Mac virus outbreaks, ClamXav is technically pointless in a Mac-only environment. But, if you’re in a combined Mac and PC environment, ClamXav is an essential security tool. While Macs cannot be affected by the thousands of available Windows viruses, Macs can still be called “home” by a good number of them. And you, the unsuspecting user, could easily send an infected file to one of your local PCs.
Cons: None significant.